The biological world is dizzyingly vast and varied, containing millions, possibly even trillions of species, and maybe as many mysteries. One such mystery is why animal phyla (groups of animals that share an evolutionary history) differ dramatically in the diversity of species they contain. The phylum Arthropoda, for example, which includes favorites like spiders, crabs, and butterflies, contains over 1.2 million different species, whereas Placozoa has only one.

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As anyone who has glimpsed both a jellyfish and a lion knows, species throughout the animal kingdom also vary widely in their biological traits, including body structure, living environment, and reproductive process. The link between biodiversity and biological traits has long beena “fundamental but unresolved problem in biology,” according to John J. Wiens, a professor of Ecology and Evolutionary Biology at the University of Arizona.

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But a new study by Wiens and his co-author Terry Jezkova, also of the University of Arizona’s Department of Ecology and Evolutionary Biology,identified five traits that make phyla significantly more likely to have a high diversity of species than ones without them.

Wiens and Jezkova began by looking at the most diverse groups of animals such as arthropods and vertebrates to figure out what they had in common. The researchers selected 18 different biological traits that have been hypothesized to play a role in increasing species diversification within a phylum, and studied the relationship of those traits to the diversification rates of phyla. Diversification rates—a measure of how quickly a phylum becomes more diverse—are a more useful measure for explaining the causes of diversity than the number of species in each phyla because phyla vary in age, so the older groups have a head start in diversifying.

The 18 traits included possession of a head, limbs, and a complex circulatory system. However, the only traits that showed a significant impact on diversification were possessing vision, living in non-marine habitat, having a skeleton, having both male and female organisms, and being parasitic. Although the most diverse phyla often share many traits, like heads and limbs, “in terms of understanding their species richness,” Wiens says, “the things that turn out to be important are just a small number of traits that seem to explain it.”

In fact, three of those five traits — living in a non-marine habitat, having a skeleton, and being parasitic — seem to have an especially significant effect on diversification rates. This may help account for why there are so many different species of parasites and why the animal kingdom is so proportionally dominated by those with skeletons.

Biodiversity plays a central role in the functioning and preservation of ecosystems. Thus, understanding how that diversity arises could influence the success of conservation efforts.“Species eat other species. So, generally, biodiversity is important for the functioning of an ecosystem,” Wiens says. “It’s also what shapes the world around us, the number of species. People worry about biodiversity. Well, this is biodiversity! The number of species. The world would be a very uninteresting place if there was only one species per phylum.”

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